The production of certain grades of polyolefins such as propylene polymers and polypropylene with desired distribution of molecular weights, or molecule chain lengths requires the addition of a chemical degradent such as a peroxide, to the polyolefin polymer. Typically, peroxide is added to the polyolefin in a mixing and extruding system in which the polymer is melted and extruded through a die head and strands of polyolefin from the die head are cut into pellets. The final molecular weight just upstream of the die head is very sensitive to the amount of chemical degradent added. As a result, during a typical production run, the plant operating personnel will submit line samples of pellets to a quality control laboratory for melt flow rate determination to measure polymer molecular weight. The results are usually available after the time of sample in from 1/2 to 2 hours.
If the results show a variation from the desired molecular weight, a change is made to the chemical degradent addition rate and the product sample is again sent for melt flow rate determination. This procedure is a time consuming process and can result in the generation of significant amounts of off-specification polyolefin products.
As will be described in greater detail hereinafter the present invention provides a method and apparatus wherein product melt flow rate is continuously monitored and adjusted on a timely basis which results in significantly less "off-spec" product being produced. Also, since a controller or computer is utilized, less manpower is required to ensure that a high grade polyolefin product is produced.
Heretofore various methods and apparatus have been proposed for controlling the molecular weight of an extruded polymer and for controlling the viscosity in a polymerization process. One example of a method and apparatus fo automatically controlling viscosity in a polymerization process is disclosed in Defensive Publication Ser. No. 723,407 published on Apr. 1, 1969. In this publication it is suggested that an analog computer be utilized for receiving a signal from a viscometer and then in response to the signal from the viscometer altering the position of a control valve controlling the flow of an additive to a polymer solution. In this publication it is noted that long time delays can result in overcontrol and it is suggested that the set point have a coarse adjustment and a fine adjustment.
It also has been proposed in U.S. Pat. No. 3,608,001 to control the conditions in an extruder reactor in response to a measured parameter, such as the melt flow in a slip stream supplied to a rheometer. Typically, the temperature in the extruder is controlled thereby to control the amount of gas degradation of the polypropylene.
In U.S. Pat. No. 3,898,209 there is disclosed a process for controlling the rheology of C.sub.3 + polyolefins by controlling the quantity of oxygen injected into a melted polymer as it is being processed in an extruder.
Also, U.S. Pat. No. 4,213,747 discloses a method and apparatus for controlling the viscosity of molten plastic material by adjusting the position of a throttling element in the melt stream of plastic material, such as polypropylene, in response to changes sensed in the viscosity of the melt just upstream of a die head with a rheometer.
Further there is disclosed in U.S. Pat. No. 4,233,255 a method for controlling the thickness or weight of extruded stock by sensing the weight with a running weigh scale and then adjusting the feed rate of a conveyor conveying extrudate to the running weigh scale.
Moreover, a method and apparatus for controlling the density of products produced from an extrusion process are disclosed in U.S. Pat. No. 4,244,897. This is accomplished by sensing the density of a continuous sheet of foam material which is being coiled in a coil and when the density sensed is different from a target value density, a comparator causes adjustment of the amount of blowing agent supplied to the extruder and controls the amount of additive added to a resin that is supplied to the extruder for controlling the extrusion process.
As will be described in greater detail hereinafter, the method and apparatus of the present invention differ from these previously proposed methods and apparatus by providing a method and apparatus for controlling the molecular weight of an extruded polymer by altering, in a forward control manner, the mixture of degradent and polymer fed to an extruder relative to a target melt flow rate related to a desired polymer molecular weight. The apparatus is a continuous in-line control apparatus which includes process models and a dead time model for closely matching the actual process signal response derived from the change in viscosity of the melt in a slip stream supplied to a rheometer after a correction signal has been supplied for altering the ratio of degradent and polymer fed to the mixing and extruding system, thereby to prevent hunting and overcontrol which would otherwise be incurred as a result of a delayed or slowly changing melt flow rate response.